fix: restore bundle_js template variable for Vite admin UI

[ecthelion77]

🐛 Bug-fix PR

🔗 Issue

Closes #4153

📌 Summary

The admin UI fails to load the Vite-bundled JavaScript because the bundle_js template variable is missing from the template context in admin_ui(). This causes a blank admin page or JavaScript errors.

🔁 Reproduction Steps

1. Start the gateway with the Vite-built admin UI
2. Navigate to the admin dashboard
3. The page loads without the main JS bundle — no interactive functionality

🐞 Root Cause

The admin_ui() handler in admin.py passes a template context dict to the Jinja2 template, but bundle_js (from get_bundle_js_filename()) was missing. The template references {{ bundle_js }} to load the correct hashed JS bundle filename, but it resolved to empty/undefined.

💡 Fix Description

Add "bundle_js": get_bundle_js_filename() to the template context dictionary in the admin_ui() handler. The get_bundle_js_filename() function already exists and resolves the Vite manifest to find the correct hashed bundle filename.

🧪 Verification

Check	Command	Status
Lint suite	make lint	✅
Unit tests	make test	✅
Coverage ≥ 80 %	make coverage	✅
Manual regression no longer fails	Admin UI loads correctly with JS bundle	✅

📐 MCP Compliance (if relevant)

• Matches current MCP spec
• No breaking change to MCP clients

✅ Checklist

• Code formatted (make black isort pre-commit)
• No secrets/credentials committed

[ecthelion77]

Suggested labels: bug, SHOULD, python, ui, frontend